Former Swiss Open champion Victor Hanescu put an end to Daniel Brands' dream week with a 6-3 6-4 triumph to progress through to the last four.
The big-serving German had already knocked 17-time Grand Slam champion Roger Federer out of the competition on the high-altitude clay of Gstaad.
However, he was unable to repeat his stunning win over the Swiss top seed and was sent home by the 2008 champion Hanescu.
Brands required treatment three games into the second set on the way to defeat in 70 minutes.
Hanescu, ranked 54th, finished with five aces and a break of serve per set.
"I'm very happy to play this event, it's of course a favourite of mine," said 32-year-old from Romania.
"I was pleased with how I played against a tough opponent. Brands doesn't give you anything out there and I'm just glad I could reach another semi."
He next takes on sixth seed Mikhail Youzhny who overcame fourth-seeded tournament debutant Juan Monaco 5-7 6-4 7-6 (9-7).
Monaco was playing his sixth quarter-final of the season, but was unable to stop Youzhny from reaching his second Gstaad semi-final in three years.
Meanwhile, any hopes of a home win in the tournament were dashed when Stanislas Wawrinka was forced to retire from his match against Feliciano Lopez.
The Spaniard was leading 6-4 2-6 4-3 when second seed Wawrinka, the 2005 finalist, decided to call it quits due to a back problem following a medical time-out.
Lopez will next face Dutchman Robin Haase after he fought back from a set down to defeat Spain's Marcel Granollers 5-7 6-3 7-6 (5).